This invention relates to an improvement in phosphate glasses whereby they have a significantly enhanced chemical durability, i.e., corrosion resistance.
Alkali phosphate glasses have low melting temperatures, moderate to high thermal expansion coefficients, and other optical properties which are highly desirable for many uses, such as in glass/metal seals such as hermetic seals, low temperature glass solders, optical glass elements such as lenses, etc. However, the ability to exploit these properties for commercial use in practical applications has been severely limited by their relatively high solubility in water and their low weathering resistance.
In the past, the chemical durability of phosphate glasses has been improved to acceptable limits by addition of alkaline earth oxides, Al.sub.2 O.sub.3, B.sub.2 O.sub.3, SiO.sub.2, and similar oxides. (See "Chemical Durability of Phosphate Glasses ", J. A. Wilder, Jr. and J. K. Johnstone, presented at the 81st Annual Meeting of the American Ceramic Society, Cincinnati, Ohio, Apr. 28-May 2, 1979, to be published.)
However, such additions unacceptably alter other properties of the base glasses. They increase the melting temperature or lower the thermal expansion coefficient, whereby the resultant glasses often no longer possess the desired properties necessary for the intended practical use.
Of course, other methods have been employed to increase chemical durability in other unrelated glass systems such as those of U.S. Pat. Nos. 3,607,321 and 4,087,511. In addition, it has been known to utilize phosphate glass-type compositions in order to achieve controlled release of a nitrogen-containing ingredient such as fertilizer. (U.S. Pat. No. 4,123,248.)
It is also noted that nitride ion has been employed in various unrelated glasses in the past. See, e.g., Loehman, J. American Ceramic Society, 62, 9-10, 491-494 (1979); Loehman et al, J. American Ceramic Society, 63, 3-4, 144-148 (1980); and Leedecke et al, J. American Ceramic Society, 63, 3-4, 190-193 (1980).